In recent years, progress has been made in the research and development of legged mobile robots modeled after animals which walk upright on two feet, such as human beings and apes, and they are increasingly expected to be used for practical purposes. The legged mobile robots which walk upright on two feet are unstable compared to crawler-type, four-legged, and six-legged robots, and have a disadvantage in that attitude control and walking control thereof are complex. However, they are advantageous in that they can flexibly adapt themselves to places with severe conditions, for example, places where an operational area includes bumps and depressions as in rough terrains and places with obstacles, discontinuous walking surfaces such as stairs and ladders, etc., and perform locomotion.
Most workspaces and living spaces of human beings are designed in accordance with their body mechanisms and behavioral patterns that they walk upright on two feet. As a result, there are so many barriers for present mechanical systems using wheels or other driving devices as moving means to move in living spaces of human beings. In order for mechanical systems, that is, robots, to help people with various human tasks or carry out the tasks in place of people and to come into widespread use in people's living spaces, moving areas of the robots are preferably the same as those of people. This is the reason why there are great expectations of putting the legged mobile robots to practical use. In order to enhance the adaptability of robots to people's living environments, it is necessary that they have a construction similar to that of human beings.
Various techniques have been proposed with respect to attitude control and stable walking of the legged mobile robots which walk on two feet, and many of them use a zero moment point (ZMP) as a criterion for stability evaluation of walking motion. The stability evaluation using the ZMP is based on d'Alembert's principle that a gravity force, an inertial force, and a moment thereof are applied by a walking system to a road surface and this moment is balanced with a ground reaction force and a ground reaction moment which are applied to the walking system as a reaction from the road surface. As a result of mechanical inference, a point where moments about a pitch axis and a roll axis are zero exists in a support polygon formed by contact points between the bottom surface of a foot and the road surface or on the sides of the support polygon, and this point is called the ZMP.
Biped walking control using the ZMP as a criterion has an advantage in that positions at which each foot hits the road surface can be determined in advance and kinematic constraints on a toe portion of each foot corresponding to the shape of the road surface can be easily taken into account. In addition, when the ZMP is used as a criterion for the stability evaluation, a trajectory, instead of a force, is used as a target of motion control, and therefore, there is higher technical feasibility. The concept of the ZMP and the application thereof as a criterion for the stability evaluation of a walking robot are described in “Legged Locomotion Robots” written by Miomir kobratovic (“Walking Robots and Artificial Legs” written by Ichiro Kato et al., published by The Nikkan Kogyo Shinbun, Ltd.).
The stability and controllability of the legged mobile robots during legged motion are affected not only by moving patterns of four limbs but also by the state of a road surface (ground surface or floor surface) on which they perform the legged motion, such as walking. This is because as long as a foot is placed on the road surface, it constantly receives the reaction force from the road surface. Accordingly, the structure of the foot which directly receives the reaction force from the road surface is extremely important in view of the stability and controllability of the legged mobile robots during the legged motion, and various proposals have been made.
For example, a structure is known in which an elastic sheet composed of rubber or the like is adhered to the foot bottom surface in order to reduce an impact which occurs when an idling leg (one of the legs which is separated from the road surface) is placed on the road surface, that is, an impact in a Z-axis direction (direction perpendicular to the foot bottom surface or direction which extends along a yaw axis). In addition, a structure in which a metal plate is adhered to the bottom surface of the elastic sheet in order to prevent the breakage and deformation of the elastic sheet is also known in the art. In addition, a structure in which a metal plate is provided on the foot bottom surface with a leaf spring therebetween in order to absorb the impact in the Z-axis direction and a structure in which a rubber material is applied to the foot bottom surface in order to prevent slipping on the road surface are also known in the art.
However, most of the above-described known foot structures are obtained by making improvements for reducing the impact from the road surface when the foot hits the road surface or preventing slipping on the road surface, and the basic shape thereof is not changed from a plate-like shape, as shown in FIG. 82(A). When a foot 920 shown in FIG. 82 is placed on a road surface 911, the entire region of the foot bottom surface is in contact with the road surface 911. In this known foot, when the ZMP is at the central position of the foot 910, as shown in FIG. 82(B), load of the robot may concentrate at this point and the foot 910 may deflect away from the road surface 911 and change the shape thereof. In such a case, there is a problem in that the contact area between the foot 910 and the road surface 911 decreases and the resistive force against the moment around the yaw axis also decreases. In addition, the shape of a contact surface between the foot sole and the road surface changes along with the change in the shape of the foot, and this leads to the change in dynamic characteristics of the legged mobile robot. As a result, the attitude of the robot becomes unstable.
The reduction in the attitude stability is not only caused by the deflection of the foot sole. Also in the case in which a bump is positioned under the central area of the foot bottom surface when the foot sole is placed on the road surface, the foot falls into a so-called seesaw state and a similar problem occurs.
In addition, since no consideration is made on the corners and side edges of the foot bottom surface, that is, a ground-contact surface of the foot sole, if the road surface has bumps and depressions, the corners and side edges may interfere with the road surface with bumps and depressions when the idling leg is placed thereon, and this may cause the robot to stumble. In addition, the robot may fall into a so-called stick-slip state where the robot repeatedly stumbles and recovers. As a result, the upper body of the robot may loose balance and the attitude of the robot may become unstable.
As an index of stability of the robot's attitude, a concept referred to herein as “resistive-force-generation effective surface” is used.
When there is only one ground-contact surface between the foot and the road surface, this surface is defined as the resistive-force-generation effective surface. In addition, when the foot is in point contact with the road surface, as shown in FIG. 83, a plane which is surrounded by lines which connect every two adjacent points is defined as the resistive-force-generation effective surface. In addition, when a ground-contact portion of the foot is frame-shaped, as shown in FIG. 84, a surface surrounded by the sides of the frame is defined as the resistive-force-generation effective surface. More specifically, the “resistive-force-generation effective surface” corresponds to a surface obtained by connecting the points where the resistive force against the moment about the yaw axis generated in the legged mobile robot is applied by the road surface.
When the ZMP moves as the legged mobile robot walks, the foot deforms and the area of the resistive-force-generation effective surface decreases. Accordingly, the resistance against the moment about the yaw axis generated due to the motion of the legged mobile robot decreases and the attitude of the legged mobile robot becomes unstable. As a result, spinning motion may occur. In addition, the change in the shape of the resistive-force-generation effective surface may cause an unexpected change in the behavior of the legged mobile robot, which leads to the reduction in the attitude stability of the legged mobile robot.
Accordingly, in the foot bottom surface of the legged mobile robot, both the static and dynamic adjustments of the surface pressure applied to the ground-contact surface are necessary. In other words, not only a pressure value but also the variation and distribution thereof must be adjusted. In addition, similarly, both the static and dynamic adjustments of friction are necessary.
These problems can be solved if the walking surface is limited to mainly flat surfaces or smooth, continuous surfaces. However, it is to be noted that the actual walking surfaces include continuous, swelling surfaces and discontinuous surfaces such as surfaces with bumps and depressions or steps, and these surfaces are also the cause of the reduction in the attitude stability of the legged mobile robots.
More specifically, when a foot is placed on a step, as shown in FIG. 85, the foot totters and support moment cannot be generated at a ground-contact portion. As a result, the behavior of the foot becomes nonlinear and its control becomes extremely difficult. In addition, the motion trajectory becomes unstable, and correction control and a movement plan must be reset.
In addition, when the foot is placed on a delicate, slippery surface, such as a carpet, as shown in FIG. 86, there is a high possibility that the ground-contact surface of the foot will slip and the motion stability of the legged mobile robot will decrease considerably.
In addition, when the foot is placed on a surface with high friction or a soft surface which easily catches the foot, as shown in FIG. 87, falling moment is generated due to the inertial force, etc., when the surface pressure, which depends on the shape of the ground-contact surface of the foot, or friction in the planar direction excessively increases. Therefore, it is necessary to adjust the frictional characteristics of the ground-contact portion.
In addition, when the foot is placed on a step, as shown in FIG. 88, in addition to the problem of the support moment described above with reference to FIG. 85, there is also a problem in that the foot may slide down when conditions of the shape of the step, or a bump, are not good or when the friction is extremely low. In addition, since such a motion is extremely fast compared to control cycles, there is a risk that suitable countermeasures cannot be implemented.
In such a case, as shown in FIG. 89, a structure like a plantar arch, for example, may be formed in the foot so as to avoid the edge of the step. However, in this structure, the plantar arch comes into contact with the edge of the step or the bump such that a resistive-force-generation effective surface 921 has a triangular shape, as shown by the hatched area in the figure, and conditions for ensuring the stability become severe. The motion performance and the stability must also be ensured on the steps.
In addition, with respect to biped walking robots, there is always a possibility of falling over, which must be avoided as much as possible. In order to avoid falling over, the development of control methods is carried out in view of how to avoid the disturbance of the balance and achieve stable motion and how to recover after losing the balance. In addition to the development of control methods, foot structures shown in FIGS. 90 to 92 are used.
FIGS. 90 to 92 are plan views showing schematic constructions of known feet. In the figures, each of reference numerals 12, 22, and 32 denotes a side surface (outer side surface) which is remote from the other foot (foot which is attached to a leg which forms a pair with a leg on which the foot shown in each figure is attached). In addition, each of reference numerals 13, 23, and 33 denotes a side surface (inner side surface) which is adjacent to the other foot; each of reference numerals 14, 24, and 34 denotes a side surface at the front of the robot; and each of reference numerals 15, 25, and 35 denotes a side surface at the rear of the robot. In addition, each of reference numerals 11, 21, and 31 denotes an attachment for attaching the foot on an ankle of the corresponding leg of the robot.
In the foot shown in FIG. 90, the outer side surface 12 is curved outward. In addition, in the foot shown in FIG. 91, the outer side surface 22 includes two planar surfaces such that the outer side surface 22 projects outward, and a vertex 26 is formed on a line where the two planar surfaces intersect. In addition, in the foot shown in FIG. 92, projections 36 and 37 are formed on the outer side surface 32 and the inner side surface 33, respectively, at the central positions thereof. The purpose of forming the outer side surfaces 12, 22, and 32 such that they project outward, as shown in the figures, is to improve the stability of the robot with respect to the outward (direction away from the other foot) rotation.
In FIGS. 90 and 91, in addition to the outer side surfaces 12 and 22, the inner side surfaces 13 and 23 may also project outward in a manner similar to the outer side surfaces 12 and 22, respectively.
In the above-described known foot structures, since the outer side surface of each foot projects outward, it can be assumed that the stability with respect to the leftward and rightward rotational moment of the robot is increased in a state before falling motion starts.
However, if the foot is constructed as shown in FIG. 90, once the falling motion starts and the robot is somewhat tilted outward (to the left or right), the contact area between the outer edge (edge between the outer side surface and the bottom surface) and the road surface is gradually shifted. More specifically, the foot starts to roll along the curve of the outer edge. In addition, if the foot is constructed as shown in FIGS. 91 or 92, the outer edge of the foot comes into point contact with the road surface at a single projecting point (the vertex 26 or a corner of the projection 36). Therefore, rotating motion around the yaw axis (axis which is perpendicular to the foot bottom surface) centered on the contact point occurs depending on the position of the gravity center of the robot in the falling motion. Generally, it is extremely difficult to predict how this rotating motion occurs.
As described above, in the known foot structures, the attitude of the robot in the falling motion is not constant, and is difficult to predict. Therefore, once the falling motion starts, it is difficult to implement controls related to the falling motion, for example, control to avoid falling over, control to reduce the impact of falling over, control to recover from falling over, etc. Accordingly, the robot cannot help but fall over, and it is difficult to cause the robot to recover by itself.
In addition, since the falling motion is not constant, in order to prevent the breakage of each part of the robot due to collision with the road surface when the robot falls over, it is necessary to increase the rigidity and the impact resistance of all of the parts which may collide with the road surface. Accordingly, there is a problem in that the cost of the robot increases.
In addition, the legged mobile robots are currently moving from the research stage to practical application, and there are still many technical problems which must be solved. For example, although the state of the road surface (whether or not it is rough, the coefficient of friction thereof, etc.) has a large influence on the attitude stability control in legged walking motion and stable walking, this is not fully understood. In addition, in biped walking robots such as humanoids, the gravity center is at a higher position and the stability region of ZMP during walking is smaller compared to four-legged walking robots. Therefore, the problem of attitude variation depending on the state of the road surface is particularly important for the biped walking robots.
When the walking motion on a road surface is considered, a walking method suitable for the state of the road surface is preferably used. Japanese Patent Application No. 2000-100708, which has been assigned to the present applicant, discloses a legged mobile robot which can perform suitable legged locomotion in accordance with the state of the road surface. In the legged mobile robot according to this publication, a surface contact sensor for determining the state of contact between a foot and a road surface and a relative-movement measurement sensor for measuring the relative movement (that is, slipping) between the road surface and the leg placed on the road surface are provided on the foot (plantar or sole) of each movable leg. Even when, for example, slipping occurs and the actual trajectory is shifted from a planned or scheduled trajectory, correction of a movement plan and motion control can be performed adaptively.
In addition, when walking motion of human beings is considered, walking motion on a normal road surface and that on a slippery road surface, such as a snowy road surface, are generally different from each other. In addition, walking motion on a wooden floor and that on a thick carpet are also different from each other. Human beings walk in accordance with the state of the road surface while observing the situation with five senses, selecting how to walk from among experimentally-learned walking methods, and performing attitude control in accordance with on the situation. In addition, human beings select shoes or the like which are suitable for the road surface on which they walk, and thereby easily adapt themselves to extreme road conditions such as snowy roads and dirt roads.
With respect to the walking stability of the robots, although the robots are required to walk on various kinds of road surfaces similarly to human beings, it is difficult for the robots to perform various walking motions similarly to human beings.
On the other hand, with respect to the relationship between the robots and the road surface, when the size and the weight of the robots are similar to those of human beings, it can be assumed that the influence of the road surface on the walking state of the robots is similar to that on the walking state of human beings.
In comparison, when the size and the weight of the robots are less than those of human beings, the influence of the state of the road surface may increase. As an example, a road surface which deforms when load is applied, such as a carpet, will be described below. When a human being walks on a carpet, even when the carpet is thick, the surface of the carpet is pressed at a region where a foot is placed and the road surface becomes stable since his or her weight is sufficiently large. In addition, the reaction force from fibers of the carpet has only a small influence on the walking motion. In comparison, when a small, light robot walks on the same carpet, a pressure applied to the surface of the carpet by a foot sole of the robot is small, and the surface of the carpet cannot be sufficiently pressed at a region where the foot is placed. As a result, a situation similar to that when a human being walks on a thick mattress occurs and the walking motion is largely influenced.
It is difficult for the robots to perform various walking patterns like human beings, and the robots cannot easily adapt themselves to the road surface on which they are walking. In addition, the robots and human beings receive different kinds of influences from the road surface.
Although the foot and the foot sole of the robots are widely researched and developed, it is currently difficult to obtain a perfect foot which can be adapted to any type of road surface from a both technical and financial point of view.
In addition, the legged mobile robots are still in the research and development stage, and their development mainly aims to increase the adaptability of the robot's foot in work environments where the road surface is limited.
Accordingly, as the legged mobile robots are transferred to practical application and product development to be used in people's living spaces, it is necessary to adapt them to various states of road surfaces.
In view of the above-described situation, the present applicant has proposed a legged mobile robot having a foot which can be replaced according to the state of the road surface in Japanese Patent Application No. 2000-167681.
In addition, the present applicant has also proposed a legged mobile robot having a foot which has a two-part structure including an instep which is connected to an ankle and a foot sole which is detachably attached to the instep such that it comes into contact with the road surface (Japanese Patent Application No. 2002-037997). In this structure, the foot sole can be replaced according to the state of the road surface. Since only the foot sole, which contributes most to the adaptation to the state of the road surface and which is worn most by coming into contact with the road surface, is replaced, many kinds of foot soles suitable for various states of road surfaces can be prepared at a low cost compared to the case in which the entire foot is replaced.
In addition, when a foot or a foot sole of a legged mobile robot is replaced, settings for suitable foot motion, ZMP trajectory, trunk motion, upper limb motion, and height of hips change. Accordingly, it is necessary to change these settings. In order to suitably change these settings, information such as the shape of the foot or the foot sole, the coefficient of friction, and the weight of the foot or the foot sole must be provided to a main controller of the robot's main body. In this case, a method may be used in which the information related to the foot or the foot sole is stored in a ROM mounted in the robot's main body and a user inputs information for identifying the new foot or foot sole.
However, in this method, information corresponding to all of the feet or the foot soles to be replaced must be stored in the ROM. Thus, if an extremely large number of feet or foot soles are prepared, the number of ROMs or the capacity of the ROM must be increased accordingly. This leads to a difficult problem if a sufficiently large space cannot be provided for accommodating the ROMs as in small legged mobile robots, and high costs are incurred if a large-capacity ROM is used. In addition, it is cumbersome for the user to input the above-described identification information each time the foot or the foot sole is replaced.
In addition, in the above-described known foot structures, although the impact in the Z-axis direction applied to the foot sole by the road surface when the foot sole is place on the road surface can be somewhat absorbed with the elastic sheet or the leaf spring, a force applied in a specific or unspecific direction along a plane perpendicular to the Z-axis direction (X-Y plane) is not taken into account. More specifically, when the road surface has bumps and depressions, a part of the foot may interfere with the surface with bumps and depressions (be caught by the surface or stumble thereon) when an idling leg is placed on the road surface, and there is a risk that the upper body of the robot will lose balance and the attitude thereof will become unstable. This problem becomes more severe when a high-speed motion is performed since the reaction force from the road surface increases. In such a case, the robot takes an emergency avoidance motion based on a software process performed by control means of the robot. However, it is advantageous in view of stability control and walking control if this problem can be avoided or eased with a hardware structure of the foot.
In addition, the foot is provided with various sensors for detecting basic information used by the main controller of the robot's main body to control the motion of each part, such as movable legs. For example, when the motion control of the robot is performed by using the ZMP as the criterion for stability evaluation, a plurality of force sensors for ZMP detection are disposed on the foot bottom surface (surface which comes into contact with the road surface) in order to measure the actual ZMP. In addition, the foot may also be provided with, for example, sensors for determining whether or not the foot is placed on the road surface, sensors for determining whether or not the foot placed on the road surface is slipping on the road surface, etc.
The detection values obtained by the sensors are A/D converted and are input to a main controller of the robot's main body. Then, the main controller calculates the actual ZMP on the basis of the detection values and performs other calculation processes, and controls the motion of each part, such as the walking motion, on the basis of the calculation results.
However, since the main controller of the robot's main body directly receives the outputs from the sensors mounted on the foot and performs necessary calculation processes including the ZMP calculation, there is a problem in that a large processing load is placed on the main controller. More specifically, a computing unit of the main controller which is mounted in the robot's main body performs complex and enormous calculations for, for example, setting the motion of the robot. Accordingly, if the computing unit of the main controller must calculate the actual ZMP on the basis of the outputs from the above-described ZMP detection sensors and process outputs from other sensors, a large calculation load is placed on the computing unit of the main controller.
In addition, in order to supply the outputs from the sensors provided on each foot to the main controller of the robot's main body, complex wiring is necessary to connect the sensors and the main controller. Furthermore, when the foot is replaced, it may be necessary to change the wiring in the robot's main body if the kind, the characteristics, the number, etc., of the sensors provided on the foot are changed. In such a case, there is a problem in that a large workload is required for replacing the foot.